1. Field of the Invention
This invention relates to an apparatus for reading an original.
2. Description of the Prior Art
In a known original reading apparatus, a transmission type of original such as a negative or positive film is illuminated by a halogen lamp, and light transmitted through the original is projected on a projection surface to form an image thereon. The projection surface is disposed, for example, above a reading means such as a CCD (charge coupled device), thereby enabling the projected image formed on the projection surface to be read. In an arrangement enabling reading of an projected image, there is a possibility of the density of the image varying depending on the type of film used as the transmission original or on the state of exposure when the image is picked up from the film. To minimize reading failures caused thereby, a control operation is carried out, which changes the voltage applied to the halogen lamp that illuminates the transmission original on the basis of a signal corresponding to the density of the original so as to enable the original to be read.
This control is performed in such a manner that, in the case of a negative film formed by over-exposure, the voltage applied to the halogen lamp is heightened to increase the quantity of light emitted from the lamp, or that, in the case of a negative film formed by under-exposure, the voltage applied to the halogen lamp is lowered to reduce the quantity of emitted light. In the case of a positive film, the control is such that the voltage applied to the halogen lamp is lowered since the transmittance of the positive film is larger than that of an ordinary negative film.
However, if, in this conventional method, the voltage applied to the halogen lamp is lowered, the value of calorification from the filament of the halogen lamp is reduced, and the temperature of the lamp wall decreases so that the halogen cycle is not smoothly effected and halogen compounds blacken the inner surface of the lamp wall by attaching thereto, resulting in a reduction in the amount of illumination light or in unevenness of light quantity distribution.